Printers operate by printing on a piece of paper. Historically, a continuous length of paper, folded fan-style, was pin-fed to a printer by use of holes along the edge of the length of paper. The pages within such fan-style paper were defined by perforations. Use of such continuous length paper increased efficiency over prior art printers which required each page be hand fed to the printer. However, the finished printed product from continuous length paper printers required each page be separated along the perforations defining each page and along the perforations defining the holed side region used for pin-feeding to the printer. This labor-intensive task resulted in printed pages with rough, perforated edges.
More recently, printers were developed in which the single top sheet of a paper stack is fed to a printer. Such printers were an improvement over the prior art because the resulting printed page had no perforations along its edges. To maximize efficiency of single sheet printers, paper trays were developed continuously to feed single sheets from the top of the paper stack to a printer. Problems arise when two or more sheets are fed to a printer simultaneously from the paper tray. Multiple sheet intake leads to paper jams, paper slippage, and to various other problems related to the print operation. The resulting waste of paper is also of concern, particularly in view of the increasing costs of materials.
To alleviate such multiple sheet intake, stack feed sheet printers often employ mechanisms adjacent their input ports which separate sheets of paper as they are taken into the printer, as shown in co-pending U.S. patent application Ser. No. 07/954,541 entitled "Paper Pick-Up System for Printers", filed Sep. 29, 1992, and subject to common ownership herewith. The disclosure of that patent application is incorporated herein by this reference. For such separator mechanisms to function, the top sheet of a paper stack must be properly aligned for feeding into the printer's input port.
To facilitate paper alignment, an input tray having a guide rail perpendicular to the printer input port is employed. The paper stack is manually placed in the input tray with one edge of the paper stack placed against the guide rail. For proper alignment and corresponding proper separator mechanism function, the top sheet must be aligned against the guide rail prior to feed into the printer input port. To ensure proper alignment, an alignment device is used to exert a bias force on the stack properly to align the sheets therein against the guide rail for feeding to the printer. However, the bias force necessary properly to align the to-be-fed top sheet of a paper stack varies with paper stack height due to the impingement of the alignment device on multiple sheets of the stack at one time, with the stack height determining at least in part the mass of paper to be urged against the guide rail. As the paper stack height decreases, the bias force must also decrease to prevent buckling of the paper.
Conventionally, alignment devices have utilized multiple force mechanisms, e.g. multiple springs, to vary the bias force as the paper stack height decreases. In one prior art embodiment, two leaf spring force mechanisms are employed. The first, larger leaf spring steers the whole paper stack toward a guide rail. The first spring is positioned opposing the guide rail across the planar expanse of the paper stack. The second, smaller leaf spring is positioned adjacent and vertically above the first spring. The second spring contacts the top sheet of the paper stack to produce a constant bias force on the top sheet against the guide rail.
In the prior art embodiment, both springs are fixedly mounted on the printer. A paper stack is placed on a paper tray that continually rises as the paper stack height decreases. Thus, each sheet in the stack is positioned adjacent the second spring when that particular sheet is the top sheet in the stack.
Problems arise when the paper tray does not raise the top sheet to a position adjacent the second leaf spring, rendering the second spring ineffective, or when paper becomes wedged between the separate first and second springs. In addition, the two springs are not mounted on the paper support, requiring a paper support adjustment and a spring adjustment for varying paper stack widths for feed to a printer. Finally, use of two force mechanisms results in more cost than use of one force mechanism because more assembly time, materials, maintenance and design are required for the two force system.